The global warming is known to be mainly caused by carbon dioxide gas which is emitted through human activities into the atmosphere from factories, industrial plants, thermal power plants, automobiles and so on. Reduction of carbon dioxide gas emission or retrieval of carbon dioxide gas existing in the atmosphere is thus recognized as one of the most important measures for protecting the environment.
Numerous systems have been proposed to reduce the emission or content of carbon dioxide gas. One of these such systems is shown in FIG. 5. The carbon dioxide fixation system of FIG. 5 is composed substantially of a CO.sub.2 isolator 6, an H.sub.2 producer 8 and a reactor 9. The CO.sub.2 isolator 6 isolates CO.sub.2 from the exhaust gas of a CO.sub.2 source 1, which is typically an industrial plant or a thermal plant, and concentrates the CO.sub.2 gas by eliminating impurities therefrom. The H.sub.2 producer 8 produces H.sub.2 gas by a known method such as electrolytic water decomposition. In the reactor 9, the CO.sub.2 gas and H.sub.2 gas react so that CO.sub.2 -fixing products such as methanol (CH.sub.3 0H), for example, are produced. Thus the CO.sub.2 gas retrieved from the atmosphere is fixed to a state harmless to the global warming, and, further, is changed to methanol which can be used as a fuel.
The above conventional CO.sub.2 fixation system has a serious drawback, that is, a large amount of electric energy is required to produce hydrogen gas by the electrolytic process. It is estimated that more than 90 percent of the total operational energy of the conventional CO.sub.2 fixation system is consumed in the hydrogen producer 8. Except in the case where the price of electricity is very low, the running cost of such CO.sub.2 fixation system is normally very high. The system has another disadvantage in that it does not reduce the CO.sub.2 emission so much as a whole. In some countries, a large part of electricity is generated thermally. In a thermal power plant, fossil fuel such as oil, coal or natural gas is burned to generate high temperature steam, through which a large amount of CO.sub.2 gas is inevitably generated.
Thus the following system is proposed. A part of the CO.sub.2 fixation system including the hydrogen producer is installed in a place (or country) where there is a stable supply of electricity at low cost and where it is generated by a method other than burning fossil fuel, such as by a hydraulic generator or by solar batteries. CO.sub.2 gas retrieved from the atmosphere and liquefied in other areas is transported by tankers or other carriers to the place, where the liquefied CO.sub.2 is reacted with hydrogen to produce methanol. The methanol is then usually transported back to the areas of the CO.sub.2 source.
The above separating system is difficult to visualize practically because the CO.sub.2 is not processed near its emission source but the liquefied CO.sub.2, and its fixed product, must be transported to a remote place, which increases the processing cost.